The invention relates to a picture display device comprising a cathode ray tube having a means for generating at least one electron beam, a display window with a display screen having phosphor patterns comprising parallel aligned phosphor lines, said display screen being provided with an index system comprising a plurality of index elements extending substantially parallel to the phosphor lines, and a means for deflecting the at least one electron beam across the display screen parallel to the phosphor lines to scan the display screen, the display device having no colour selection electrode in front of the display screen, and the display device comprising means for imparting video information to the at least one electron beam, the image being written in a sequence of video lines.
Picture display devices of the type described above are also called xe2x80x9cindex tubesxe2x80x9d or xe2x80x98indexxe2x80x99 display devices.
In such known index display devices, the electron beamxe2x80x94when impinging on an index element of an index systemxe2x80x94generates an index signal which is indicative of the position of the electron beam with respect to said index element and/or of the shape of the electron beam. Such an index signal is measured and subsequently used in a control loop acting on the deflection and/or formation of the electron beam in order to correct the trajectory and/or shape of the electron beam when it deviates from its nominal trajectory and/or shape. The great advantage of these types of display devices over the conventional xe2x80x98shadow maskxe2x80x99 tubes is that such index tubes do not need and do not use a colour selection electrode placed immediately in front of the display screen. Such colour selection electrodes absorb part of the energy of the electron beam(s) and increase the complexity and weight of the device.
Although the known index devices work satisfactorily in many circumstances, accurate control of the electron beam position and/or shape requires a small spot size perpendicular to the scanning direction of the electron beam(s) so as to prevent overlap with neighbouring phosphor lines, which results in a loss of colour purity and thereby a loss of image quality.
It is an object of the invention to provide an index display device having an improved image quality.
To this end, the display device in accordance with the invention is characterized in that the at least one electron beam is scanned across phosphor lines, the phosphor lines are arranged in the colour sequence A-B-C-B-A-B-C, etc., where A, B and C stand for red, green and blue phosphors or any mutation of said colours, wherein, during a scan, the at least one electron beam substantially scans a single phosphor line, and, in operation, the video lines overlap such that each B phosphor line is written in a single video line and each A and C phosphor line is written by two video lines.
In known index tubes, the index system is deposited on the inner side of the display window. For instance, a line of phosphor is deposited between each pair of index elements. If an electron beam is scanned across the phosphor line, along the direction of the phosphor line, signals are induced in or by the index elements that can be used to generate feedback signals to keep the beam on track. A video frame is written one triplet at a time. Each video line follows the previous video line.
In operation, the video lines are addressed in the sequence {Ai, Bi, Ci}, {Ai+1, Bi+1, Ci+1}. The inventors have realised that one of the major problems is the required small size of the electron beam perpendicular to the phosphor lines. If the electron beam spot size grows larger than the width of the phosphor line, it will not only hit the addressed phosphor, but also the neighbouring phosphor, which will result in a loss of colour purity. The demands on the spot size limit the performance and image quality of the index tube, both in resolution and in brightness.
The invention allows an increase of the maximum allowable spot size while maintaining the overall resolution. Instead of using the A-B-C,A-B-C,A-B-C structure, the odd triplets are inverted, giving an A-B-C,C-B-A,A-B-C structure, etc. The similar neighbouring phosphor lines are combined to one phosphor line, resulting in the sequence A-B-C-B-A-B-C, etc. Most advantageously, the sequence is RGBGRGB (RED-GREEN-BLUE-GREEN-RED-GREEN-BLUE, etc.) i.e. B=Green, although other sequences (wherein, for instance, B=Blue) may be possible. This means that, at constant pitch, the average width of the phosphor lines may be increased and, at constant width of the phosphor lines, the pitch can be decreased from p to ⅔p. Since the A and C video components of two video lines are now written on the same phosphor line, effectively the resolution of these colours is halved. The resolution of the B colour is, however, not changed. The overall resolution is, however, not equally dependent on the three colours. By choosing the colour for B on which, in a particular device, the perceived resolution is most dependent (which is usually green), the overall resolution as perceived by the viewer will not deteriorate much as compared to the perceived overall resolution of known devices, while the colour purity and/or brightness will greatly improve.
Preferably, sequential video lines are addressed in operation in accordance with the scheme {A,B,C}{C,B,A}{A,B,C}, etc.
Such an addressing scheme is preferred because it is a relatively simple scheme both in addressing and in deflection.
The device for generating an electron beam may comprise a device which generates a single electron beam, but preferably the means for generating electron beams are provided for generating a central and two outer electron beams spaced apart in a direction transverse to the phosphor lines.
Using three instead of one electron beam reduces the overall scan rate (by one third) and thereby reduces the complexity of the device.
Preferably, video information relating to colour B is imparted to said central beam.
This reduces the complexity of the addressing scheme because the central beam is always directed to one colour.
Preferably, the display device comprises
means for alternating the position of the outer electron beams with each video line and imparting video information to colour A to one of the outer electron beams and video information to colour C to the other one of the outer electron beams, or
means for alternately imparting, with each video line, the video information to colours A and C to the outer electron beams.
The two outer beams that do not write the central (2) phosphor line will now alternate each video line between writing the one and the other colour, while the video lines are written {Ai,Bi,Ci}, {Ci+1,Bi+1,Ai+1}. The two (for instance, R and B) beams are alternated by one of the two following modes
The video signal imparted to the three electron beams is unaltered, and the relative positions of the outer electron beams are moved each video line with respect to the central beam.
The two outer electron beams keep their position with respect to the central beam and the A and C video signal are swapped every video line.
It is noted that devices are known with a single electron beam (such as from U.S. Pat. No. 2,809,233) which is simultaneously scanned across three phosphor lines by wobbling the electron beam (i.e. displacing it in a direction perpendicular to the phosphor lines) very fast across the phosphor lines, so that the electron beam excites all phosphor lines during each scan. In at least one example the arrangement of the phosphor lines in this known device is the same as described above. Such a device is, however, very complicated and accurate indexing is difficult because of the wobbling. Also the different phosphor lines are separated by guard bands. These guard bands lead to a loss of intensity, because no light is produced when the electron beam passes across the guard bands. In the present invention, the electron beam(s) are scanned substantially across one phosphor line during each scan. The efficiency is therefore higher and the accuracy of indexing is improved, both of which effects lead to an improved image reproduction.
Devices are also known (such as from UK Patent specification 808,138) in which a phosphor screen is used and a colour selection electrode (a grid) is placed in front of the phosphor screen. The grid comprises two sets of wires between which, in operation, an alternating potential is supplied, which deflects the electron beams. Such a construction is complicated, requiring a colour selection electrode of intricate design and high mechanical stability and precision as well as means for providing alternating high voltage potentials. The grid must also be accurately positioned (in a stable manner) with respect to the phosphor lines.
The above-mentioned and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.